More than seventy years after its publication, Hans Kohn's 1944 The Idea of Nationalism is still regarded as a ground‐breaking contribution to the study of nationalism. This essay is aimed to highlight a significant theme in this work which has largely gone unnoticed, namely, the pivotal role of religion and secularism in Kohn's account of nationalism, and especially, in his persistent struggle for a ‘perfect’ nationalism. Kohn's conception – and personal experience – of the relationship of nationalism and religion will be examined through several stages of his turbulent life. First, as a young Zionist in Prague, when he parlayed Martin Buber's Zionist creed into an ethnic concept of nationalism. Then, in Kohn's journalistic writing in the 1920s and in his first theoretical works on nationalism in the years 1929–1942. Finally, Kohn's more mature and crystallized account of nationalism in his 1944 book will be revisited from the perspective of the nationalism–religion relationship.